fix: persist dashboard metrics and count versions instead of repos

Metrics (downloads, uploads, cache hits) were stored in-memory only
and reset to zero on every restart. Now they persist to metrics.json
in the storage directory with:
- Load on startup from {storage_path}/metrics.json
- Background save every 30 seconds
- Final save on graceful shutdown
- Atomic writes (tmp + rename) to prevent corruption

Artifact count on dashboard now shows total tags/versions across
all registries instead of just counting unique repository names.
This matches user expectations when pushing multiple tags to the
same image (e.g. myapp:v1, myapp:v2 now shows 2, not 1).

nora-registry/src/main.rs

@@ -336,7 +336,7 @@ async fn run_server(config: Config, storage: Storage) {
         start_time,
         auth,
         tokens,
-        metrics: DashboardMetrics::new(),
+        metrics: DashboardMetrics::with_persistence(&storage_path),
         activity: ActivityLog::new(50),
         audit: AuditLog::new(&storage_path),
         docker_auth,
@@ -387,6 +387,16 @@ async fn run_server(config: Config, storage: Storage) {
         "Available endpoints"
     );
 
+    // Background task: persist metrics every 30 seconds
+    let metrics_state = state.clone();
+    tokio::spawn(async move {
+        let mut interval = tokio::time::interval(std::time::Duration::from_secs(30));
+        loop {
+            interval.tick().await;
+            metrics_state.metrics.save();
+        }
+    });
+
     // Graceful shutdown on SIGTERM/SIGINT
     axum::serve(
         listener,
@@ -396,6 +406,9 @@ async fn run_server(config: Config, storage: Storage) {
     .await
     .expect("Server error");
 
+    // Save metrics on shutdown
+    state.metrics.save();
+
     info!(
         uptime_seconds = state.start_time.elapsed().as_secs(),
         "Nora shutdown complete"